I have a 85 and an 86 Plus 8 with the LT77 box and Smiths gauges. Ensure your cable is not binding as the pressure will cause it to turn inconsistently. Even if it is new, it may still bind if the routing is problematic. Also, both in the speedo and the cable, the older lubricants start to harden up causing resistance which can lead to erratic reading.
Cheers,
